Taenia solium cysticercosis is the main cause of acquired epilepsy in developing countries. The availability of an accurate EITB assay for cysticercosis, 4 provided a sensitive and specific marker for T. solium infection, and demonstrated that in many endemic zones 20% of more of the general population may carry specific antibodies. Whereas in clinical center neurocysticercosis is commonly seen in epileptic patients, in endemic rural areas moist infected persons are neurologically asymptomatic. In these areas, up to 15% of seronegative individuals may show calcified cysts on cerebral computed tomography (CT). All these findings were assumed to support the idea of a largely asymptomatic infected human population, with minimal changes through time. However, we have recently found in three different studies that a significant proportion of seropositive individuals revert to seronegative in one year or less. This previously unrecorded phenomenon means that T. solium is a very dynamic infection in which the majority of newly infected cases will only develop a transient serological antibody reaction. The biological significance of this "transient" seropositivity as a marker of exposure, a marker of current infection, or an indication of protection from further infection, is not known. Under this TMRC program approach, this component will primary emphasize the use of new techniques (immunological assays, imaging studies, GPS/GIS) towards the elucidation of the biological significance of instant seropositivity.